change directory structure to match index

Signed-off-by: Michel-FK <michel.stempin@funkey-project.com>

docs/developer_guide/hardware_reference/index.md

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+The **FunKey S** is the world's smallest foldable retro-gaming console.
+
+Holding a world's record is great, but we wanted a device that was
+_actually_ playable and that still could be attached to a keychain.
+
+Reaching this goal required more than just the usual job to shrink
+everything you can: it required a radical change in the design itself.
+
+## Foldable Design
+
+The idea to use a foldable clamshell design came up rapidly as the
+only way to get _both_ a small form factor when closed, and offer the
+largest screen and keypads while playing.
+
+But this solution also brings a lot of challenges, as the required
+hinge make connections between the 2 halfs of the shell more
+difficult.
+
+However, this has been done before in the classic GameBoy Advanced SP,
+ or in many clamshell cellphones from some years ago, so why can't we?
+
+## Design Goals
+
+Passed this first design decision, we fixed ourselves some additional
+goals in order to provide the user a good gaming experience:
+
+ - have a CPU that is powerful enough to be able to emulate most
+   retro-gaming consoles, up to the PS1
+
+ - have a good quality screen with a resolution large enough to
+   display even the games featuring small texts
+
+ - have a set of keys and pads large enough and complete enough to be
+   able to play comfortably
+
+ - have a built-in audio capability (who can play without sound?)
+
+ - be able for a 6-year old child to add games and emulators
+
+ - as an extremely nomad device, be able to stop playing just by
+   closing the console, and restart when opening it just where you
+   left (we call this feature "**_Instant Play_**")
+
+## Design Constraints
+
+Besides the constraint brought by the hinge listed above, the main
+difficulty is due to the maximum device dimensions that should not
+exceed those of a (large) car key.
+
+And because of the foldable design, the most constrained dimension is
+thickness: having a folding screen also means doubling the plastic
+enclosure around the screen and the other internal parts.
+
+In order to meet these constraints, we had to take more
+(heart-breaking) decisions:
+
+ - a CPU with external DRAM chips would take too much real-estate on
+   the PCB, so we had to find a CPU with integrated DRAM
+
+ - an audio Jack would be very difficult to integrate, so we decided
+   to use an internal speaker instead
+
+ - an USB C connector is too thick and large, so we used a Micro USB
+   connector instead
+
+ - a Micro SD connector with external access would be too large, as
+   the required "push/push" mechanism, plus the mechanical design to
+   make sure the card does not get loose make the solution much larger
+
+ - because of real-estate constraints on the PCB as well as power
+   consumption constraints, we decided to not integrate Wi-Fi and/or
+   Bluetooth
+
+ - the only keypad technology able to meet our thickness goal is using
+   integrated tactile switches
+
+ - battery capacity had to be adjusted to the maximum available
+   internal volume, using standard battery dimensions
+
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+includes/glossary.md
+--8<--